This invention relates to a microwave circuit and, more particularly, to a microwave amplifier implemented by field effect transistors.
A typical application of the microwave amplifier is a power control for a transmitter incorporated in a wireless telephone. The output power of the wireless telephone is controlled by using the microwave amplifier, and a field effect transistor is the essential circuit component of the prior art microwave amplifier circuit. The gate voltage or the drain voltage is varied for controlling the output power as described hereinbelow.
FIG. 1 shows a typical example of the amplifier circuit. An input terminal 601 and an output terminal 602 are connected to the gate electrode of a field effect transistor 611 and the drain node of the field effect transistor 611, respectively. The field effect transistor 611 serves as a main amplifier. A power source 641 is connected through a choking coil 613 to the drain node of the field effect transistor 611, and the controller 612 is connected through a resistor 663 to the gate electrode of the field effect transistor 611. The power source 641 supplies electric current through the choking coil 613 to the drain node of the field effect transistor 611 at all times. The field effect transistor 611 discharges part of the electric current through the source node thereof, and remaining electric current flows into the output terminal 602. The controller 612 varies the potential level at the gate electrode of the field effect transistor 611, and, accordingly, controls the gain of the field effect transistor 611. As a result, the field effect transistor 611 varies the electric power at the output terminal 602.
FIG. 2 shows another example of the amplifier circuit. The second prior art amplifier circuit also includes a field effect transistor 711. An input terminal 701 and an output terminal 702 are connected to the gate electrode of the field effect transistor 711 and the drain node of the field effect transistor 711, respectively. A power source 741 is connected through a dc-to-dc converter 739 and a choking coil 713 to the drain node of the field effect transistor 711, and a gate bias terminal 712 is connected through a resistor 763 to the gate electrode of the field effect transistor 711. The dc-to-dc converter 739 varies the potential level at the drain node of the field effect transistor 711 so that the field effect transistor 711 controls the electric power at the output terminal 702. The dc-to-dc converter 739 is implemented by MOS (Metal-Oxide-Semiconductor) field effect transistors, which are fabricated on a silicon substrate. On the other hand, the field effect transistor 711 is usually fabricated on a compound semiconductor. A field effect transistor with a channel region of silicon is hereinbelow referred to as xe2x80x9cfield effect silicon transistorxe2x80x9d, and a field effect transistor with a channel region of compound semiconductor is hereinbelow referred to as xe2x80x9cfield effect compound semiconductor transistorxe2x80x9d.
Yet another example of the prior art amplifier circuit is disclosed in Japanese Patent Publication of unexamined Application No. 9-64757. The third prior art amplifier circuit detects an output electric power, and changes the drain voltage at drain nodes of field effect transistors incorporated in a power amplifier between two potential levels depending upon the output electric power. The drain node is connected to a variable dc voltage circuit or a dc-to-dc converter, and the two potential levels are selectively supplied to the drain node of the field effect transistor. With the two potential levels selectively supplied to the drain node, the power amplifier is expected to offer linear input-to-output characteristics. A pulse width modulation converter is usually used for the dc-to-dc converter, and MOS field effect silicon transistors form the pulse width modulation converter. MES (Metal-Semiconductor) field effect compound semiconductor transistors form the power amplifier. However, bipolar transistors, MOS field effect silicon transistors and heterobipolar transistors are available for the power amplifier.
Following problems are encountered in the above-described prior art amplifier circuits. The problems inherent in the first prior art amplifier circuit are a low efficiency and a distortion. This is because of the fact that the controller 612 varies the biasing voltage at the gate electrode of the field effect transistor 611.
The problem inherent in the second prior art amplifier circuit is a low power efficiency, a difficulty in integration on a single semiconductor chip and poor response characteristics. The low efficiency, the difficulty and the poor response characteristics are derived from the dc-to-dc converter 739. As described hereinbefore, the dc-to-dc converter 739 is implemented by the field effect silicon transistors, and are low in switching speed. The second prior art amplifier circuit is expected to control the output power at a highspeed. However, the dc-to-dc converter can not respond to the high-speed power control due to the low switching speed of the field effect silicon transistors. This results in the poor response. The efficiency of the dc-to-dc converter 739 is of the order of 85 to 90 percent, and the field effect silicon transistors incorporated therein decrease the efficiency together with decrease of the power voltage. As a result, the second prior art amplifier circuit can not achieve a high power efficiency. It is difficult to integrate the field effect compound semiconductor transistor 711 and the field effect silicon transistors of the dc-to-dc converter on a single semiconductor chip. If the field effect compound semiconductor transistor is replaced with a field effect silicon transistor, a silicon substrate is shared between the field effect silicon transistor and the field effect silicon transistors of the dc-to-dc converter 739. However, the field effect silicon transistors are designed to have a wide channel in order to decrease the series resistance. This means that the field effect silicon transistors and the dc-to-dc converter 739 occupy wide real estate on the silicon substrate. For this reason, the second prior art amplifier circuit requires a large-sized silicon chip. The large-size silicon chip makes the integration difficult from the viewpoint of production cost.
A problem inherent in the third prior art amplifier circuit is high production cost. The third prior art amplifier circuit changes the drain voltage between two potential levels depending upon the detected output power. The change of the drain voltage aims at a constant power gain and, accordingly, good linearity over a wide output power range. The set drain current is made constant in a lower power operation mode for the good linearity. The output power is detected by using an output power envelope detecting circuit. The output power envelope detecting circuit is a large electric circuit, and is expensive. This results in the high production cost of the third prior art amplifier circuit.
It is therefore an important object of the present invention to provide a microwave amplifier circuit, which is high in efficiency in a low output power operation, small in distortion and easy for integration on a single semiconductor chip.
In accordance with one aspect of the present invention, there is provided a microwave amplifier circuit comprising a field effect transistor serving as a main amplifier and a drain bias controlling circuit including a power source for generating an electric power and first heterojunction field effect transistors formed of compound semiconductor and serving as switching units electrically connected between the power source and a drain node of the field effect transistor and selectively changed between on-state and off-state so as to vary a drain voltage applied to the drain node.